In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has drastically progressed and development for applying a TFT to an active matrix display device has been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher electric field effect mobility (also referred to as mobility, simply) than a conventional TFT using an amorphous semiconductor film, high speed operation is possible. Therefore, it has been attempted that a pixel, which has been controlled by a driver circuit provided outside a substrate, is controlled by a driver circuit provided over the same substrate as the pixel.
A substrate used for a semiconductor device is expected to be a glass substrate rather than a quartz substrate or a single-crystal semiconductor substrate in terms of cost. However, a glass substrate is inferior in heat resistance and Easy to be deformed due to heat. Therefore, when a semiconductor film formed over a glass substrate is crystallized to form a TFT using a polycrystalline semiconductor film, laser annealing is often employed in order to prevent the glass substrate from being deformed due to heat.
Compared with another annealing method which uses radiation heat or conduction heat, the laser annealing has advantages that the process time can be shortened drastically and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that thermal damage is hardly given to the substrate. The laser annealing method described here indicates a technique to recrystallize an amorphous layer or a damaged layer formed in a semiconductor substrate or a semiconductor film, and a technique to crystallize a non-single crystal semiconductor film formed over a substrate. Further, a technique applied to planarization or modification of the surface of a semiconductor substrate or a semiconductor film is also included.
Laser oscillators used for the laser annealing can be roughly categorized into pulsed laser oscillators and continuous wave laser oscillators according to the oscillation method. In recent years, it has been known that the size of a crystal grain formed in a semiconductor film becomes larger when using a continuous wave laser oscillator such as an Ar laser or a YVO4 laser than when using a pulsed laser oscillator such as an excimer laser in crystallizing the semiconductor film. When the size of the crystal grain in the semiconductor film becomes larger, the number of crystal grain boundaries in a channel region of a TFT formed using this semiconductor film decreases, and the carrier mobility becomes higher, so that a more sophisticated device can be developed. For this reason, a continuous wave laser oscillator is attracting attention.
In general, a laser beam which is used for laser annealing of a semiconductor film has a linear spot shape and the laser annealing is conducted by moving the linear spot of the laser beam on the semiconductor film. By shaping the laser beam into the linear spot, the area annealed by the laser beam at one time can be made larger. In this specification, laser beams having a linear shape and a rectangular shape on an irradiation surface are referred to as a linear beam and a rectangular beam, respectively. It is to be noted that the term of “linear” herein used does not mean a line in a strict sense but means a rectangle having a high aspect ratio (for example, aspect ratio of 10 or higher (preferably 100 to 10000)). The laser beam is shaped into a linear spot because energy density required for sufficient annealing to an irradiation object can be secured. Therefore, as long as sufficient annealing can be conducted to the irradiation object, the laser beam may be shaped into a rectangular or planar spot. In the future, there is possibility that laser annealing is conducted with a planar beam.
On the other hand, when a silicon film having a thickness of several tens to several hundred nm, which is generally used in a semiconductor device, is crystallized with a YAG laser or a YVO4 laser, a second harmonic having a shorter wavelength than the fundamental wave is used. This is because the second harmonic has higher absorption coefficient of a laser beam to a semiconductor film than the fundamental wave and the semiconductor film can be crystallized effectively with the second harmonic. The fundamental wave is rarely used in this step.
An example of this step is as follows: a CW (continuous wave) laser beam converted into a second harmonic having a wavelength of 532 nm and a power of 10 W is shaped into a linear beam spot with a length of about 300 μm in a major-axis direction and about 10 μm in a minor-axis direction and this linear beam is moved in the minor-axis direction, thereby conducting laser annealing. Thus, crystallization is performed. The width of a region where a large grain crystal is obtained by one scan is approximately 200 μm (hereinafter, a region where a large grain crystal is observed is referred to as a large grain region). Therefore, in order to crystallize the whole surface of the substrate by laser annealing, it is necessary to carry out laser annealing in such a way that a position from which a laser beam is moved is displaced in a major-axis direction of a beam spot by a width of a large grain region obtained by one scan of the beam spot.
The present applicant has already made the invention in which a semiconductor film is irradiated with a laser beam shaped into a linear beam at or in the vicinity of an irradiation surface, and filed patent application (see, Japanese Patent Application Laid-Open No. 2003-257885).